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→Determine the infiltration water storage depth of the practice
*d<sub>f</sub> = Depth of filter media (m)
*d<sub>f</sub> = Depth of filter media (m)
*n<sub>f</sub> = Porosity of filter media}}<br>
*n<sub>f</sub> = Porosity of filter media}}<br>
For practices with the underdrain perforated pipe elevated off the bottom, infiltration water storage is provided by the storage reservoir, d<sub>r</sub> (i.e. depth below the invert of the underdrain perforated pipe), and only the portion that can reliably drain by infiltration within the specified inter-event drainage time, t. So the infiltration water storage depth of the practice can be calculated as:
For practices with an underdrain where the perforated pipe is elevated off the bottom, infiltration water storage is provided by the depth of the storage reservoir below the invert of the underdrain perforated pipe, d<sub>r</sub>, and only the portion that can reliably drain by infiltration within the specified inter-event drainage time, t. So the infiltration water storage depth of the practice can be calculated as:
<math>d_{i}= f' t </math>
<math>d_{i}= f' t </math>
{{Plainlist|1=Where:
{{Plainlist|1=Where:
*f' = [[Design infiltration rate]] of underlying native soil (m/h)
*f' = [[Design infiltration rate]] of underlying native soil (m/h)
*t = [[Drainage time]] (h), time required to fully drain the active storage components of the practice (i.e. surface ponding and infiltration water storage depths), based on local criteria or long term average inter-event period for the location}}<br>
*t = [[Drainage time]] (h), time required to fully drain the active storage components of the practice (i.e. surface ponding and infiltration water storage depths), based on local criteria or long term average inter-event period for the location}}<br>
For practices with the underdrain perforated pipe installed on the bottom and connected to a riser (e.g., standpipe and 90 degree coupling), infiltration water storage is provided by the storage reservoir depth between the inverts of the riser outlet (i.e invert elevation of the 90 degree coupling) and reservoir bottom, and is calculated the same way as above.<br>
For practices with an underdrain where the perforated pipe is installed on the bottom and connected to a riser (e.g., standpipe and two 90 degree couplings), infiltration water storage is provided by the storage reservoir depth between the inverts of the riser outlet (i.e invert elevation of the 90 degree coupling) and reservoir bottom, and is calculated the same way as above.<br>
To boost drainage performance on fine-textured, low permeability soils, consider designing storage reservoirs even deeper than those calculated using the above approach, that many not fully drain between storm events (i.e. includes inactive water storage), which increases hydraulic head and thereby, infiltration rate at the base of the practice. See [[Low permeability soils]] for more information.
To boost drainage performance on fine-textured, low permeability soils, consider designing storage reservoirs even deeper than those calculated using the above approach, that many not fully drain between storm events (i.e. includes inactive water storage), which increases hydraulic head and thereby, infiltration rate at the base of the practice. See [[Low permeability soils]] for more information.